Electretized fiber sheets are widely used because they exhibit high dust-collecting performance with low pressure loss when used as air filters. As a method for producing such electret fiber sheets, a method in which a high voltage is applied to fiber sheets such as a nonwoven web that are made of synthetic fiber and thus the fiber sheets are electretized by corona discharge is proposed (see Patent Literature 1).
Electret fiber sheets produced by this corona discharge method exhibit higher dust-collecting performance compared with fiber sheets that are not electretized. However, the method for producing electret fiber sheets by the corona discharge method is a production method in which mainly only the surface of a fiber sheet is electrically charged and which leaves it difficult to charge the inside of the fiber sheet with electric charge, and thus it is difficult to say that the method achieves sufficient electretization.
In view of this, as electretization methods to solve such a problem, methods in which to electrically charge a fiber by subjecting it to water contact have been proposed. Specifically, proposed are: a method in which a fiber sheet to be used is electretized by impinging on the whole surface of the fiber sheet high pressure water spouted from a mouthpiece of a so-called water jet punch (WJP) in which the spinning nozzle has holes arranged in line at narrow spacings, such that the fiber sheet has positive polarity charge and negative polarity charge evenly mixed (see Patent Literature 2); and a so-called hydrocharging method such as a method in which a fiber sheet is passed over a nozzle having a slit shape while the nozzle sucks water to thereby allow the water to permeate into the fiber sheet, such that the fiber sheet has positive polarity charge and negative polarity charge evenly mixed (see Patent Literature 3).